Method for thermochemically treating a part while masking a portion and corresponding mask

ABSTRACT

A method for thermochemically treating a part while masking a portion and corresponding mask are provided. The method includes the steps of providing a mask comprising a body with a seat, at least a deformable sealing washer located in the seat, and a tightening bushing, the body having a cavity, placing a first portion of the part in the cavity, a second portion of the part being located in a passage in the sealing washer, and a third portion of the part being located outside the mask, moving the tightening bushing to its tightened position so that the sealing washer is deformed and applied against the second portion of the part, and applying a thermochemical treatment to the third portion of the part.

This is a divisional application of U.S. patent application Ser. No.14/438,851 filed Apr. 27, 2015, the entire disclosure of which is herebyincorporated by reference herein.

The present invention relates to the manufacture of parts with improvedproperties, such as wear resistance and corrosion resistance. Thetreatment applied to the parts in order to improve these properties maybe a hardening or a protection treatment through the diffusion of atomspecie(s).

The invention is particularly applicable to the manufacture of parts foruse in light water reactors (LWR), including pressurized water reactors(PWR) and boiling water reactors (BWR). The parts comprise, forinstance, tubes such as cladding tubes for neutron absorbing rods ornuclear fuel rods, flux thimbles for nuclear instrumentation of thereactor core . . . , bars such as neutron absorbing bars, support pins,positioning pins, end plugs . . . , bolts such as edge bolts for thebaffle assembly of the reactor vessel . . . , springs such as holddownsprings of the nuclear fuel assemblies, etc.

BACKGROUND

In PWR, neutron absorbing rods are usually grouped in control clusters.Among the control clusters of a nuclear reactor, some are dedicated tocontrolling the core reactivity in normal operation of the nuclearreactor. They may be frequently displaced in stepping mode to insert theneutron absorbing rods into or extracted them from the guide thimbles ofthe nuclear fuel assemblies of the nuclear reactor. Stepping motions maycause distributed wear on the neutron absorbing rods due to contact withguides against which they rub.

The other control clusters stay stationary in the guides in the upperpart of the nuclear reactor during normal operation. When a rapid returnto the sub-critical state is required, these clusters are simultaneouslyand fully inserted by gravity in the reactor core, i.e. the neutronabsorbing rods are inserted in the corresponding guide thimbles of thecorresponding nuclear fuel assemblies. Flow induced vibrations may causelocalized wear on the neutron absorbing rods of the stationary controlclusters:

-   -   at the contact levels in the guides, and    -   at their lower end due to contact of the lower portion of the        cladding and the lower end plug with the upper portion of the        guide thimbles of the nuclear fuel assembly (“tip-wear”).

The possible consequences of wear, whether from stepping motions orflow-induced vibrations, are:

-   -   cladding wear-through with potential contamination of the water        of the primary coolant system of the nuclear reactor by the        neutron absorbing material enclosed in the cladding, and    -   neutron absorbing rod mechanical failure due to reduction of the        mechanical strength of the cladding.

The frequency and amplitude of the movements of some of the controlclusters, especially when the reactor is operated in the load followmode (suivi de charge in French), and the frequency and amplitude ofvibration of some of the neutron absorbing rods, especially for controlclusters remaining in a stationary position, are such that it isnecessary to check frequently and often prematurely replace a number ofcontrol clusters given the wear resulting from friction.

To prevent this wear, it was proposed to harden the outer surface of thecladdings through nitriding and/or carburizing treatments. U.S. Pat. No.4,873,117, EP-446,083, EP-537,062 and EP-801,142 describe the steps ofsuch treatments especially under plasma conditions.

Such treatments can efficiently protect the claddings and the lower endplugs of neutron absorbing rods against wear and corrosion.

As disclosed e.g. in U.S. Pat. No. 4,873,117, the claddings to betreated are first cleaned, then fitted with their lower plugs. A thermalbuffer body, which can be constituted by a slug of stainless steel, isplaced in each cladding and then a temporary upper plug is fixed on eachcladding. The temporary upper plug is used for holding the claddingduring treatment and temporarily closing off the open end of thecladding and then avoiding pollution of the treatment atmosphere by aircontained in the cladding. The claddings are placed in a treatmentenclosure and their upper portions close to the upper plugs areadvantageously masked.

After treatment, the claddings are taken out of the enclosure. Thetemporary upper plugs are removed, and the claddings are loaded withneutron absorbing material and a final upper plug is welded on the freeend of each cladding.

Masking of the upper portion of the cladding avoids treatment of thecladding material in this zone. Indeed, such a treatment would have animpact on the material features and on the properties of this upperportion which is to be welded to the final upper plug. As an example,carbonitriding the upper portion might lead to carbide and/or nitrideprecipitates during welding to the upper plug, thus leading to a lowerresistance of the cladding to strain and corrosion.

More generally portions of parts which have to be treated, e.g.oxidized, nitrided, carbonitrided . . . shall be masked so as to avoidmodifying material features and properties and hindering the subsequentmanufacturing operations applied to these portions: mechanical forming,stamping, welding, mechanical machining, threading . . . .

Various masking methods are known and used for shielding portions ofparts which need to be treated.

In particular, as described above, solid masks have been used fortreatment involving plasmas. Such a solid mask receives a portion to beshielded from treatment with a fitting clearance. This clearancecorresponds to the space needed to ensure the correct mounting of themask on the portion and needs to have a thickness lower than the Debyelength. The Debye length is the scale over which mobile charge carriers(e.g. electrons) screen out electric fields for specific plasmaconditions. In other words, the Debye length is the distance under whichsignificant charge separation can not occur.

The thickness of the clearance being lower than the Debye length, theplasma conditions are not present in the clearance, so that a partportion surrounded by the solid mask with said clearance will beshielded from the treatment applied to the rest of the part locatedoutside the solid mask.

However, such solid masks are inefficient if the nominal fittingclearance, which can not be lower than the cumulated manufacturingtolerances of the masks and of the portions to be shielded, is greaterthan the Debye length. In addition, even if the nominal fittingclearance is lower than the Debye length, the solid masks have proved tobe inefficient at least for plasma carbonitriding treatments, the maskedportion experiencing depassivation and/or hardening despite the presenceof the solid mask.

Solid masks with clearance have also proved to be inefficient for oxygendiffusion treatments, in the plasma or gas phases, of hafnium claddingsor hafnium solid bars used as neutron absorbing rods in controlclusters, or of zirconium alloy claddings used for nuclear fuel rods astaught for instance in WO-2009/081013.

WO-02/066,698 discloses a solid mask used to shield a portion of a partfrom a carburizing treatment. The solid mask has a thermal expansioncoefficient lower than the thermal expansion coefficient of the part tobe treated. Thus, the clearance between the portion to be shielded andthe solid mask will decrease during the carburizing treatment. However,this requires high treatment temperatures, e.g. around 500° C. to 900°C. depending on the materials of the part and of the mask, and there isa high risk of damaging the outer surface of the part, or even to modifyits geometry if the part is a thin-walled tube, through contact withsaid mask.

This solid mask can therefore not be used e.g. for plasma carbonitridingof neutron absorbing rod claddings. Indeed, the materials used for theneutron absorbing rod claddings (mainly stainless steels or nickel-basedalloys) have thermal expansion at the treatment temperatures which arelower than or at best of the same order of magnitude than themanufacturing tolerances of the claddings. There is thus no guaranteethat the masked portions of all the claddings of the treated batch areprotected in a homogeneous manner. Moreover, the outer surfaces of thecladdings shall not be damaged by fixing or removal of the masks orduring the treatment, which prevents the use of smaller clearance.

SUMMARY OF THE INVENTION

One goal of the invention is to overcome these drawbacks by providing amethod for thermochemically treating a part, which method provides foran efficient masking of a portion of the part, even under plasmacarbonitriding or oxidation treatment, with lower risks to damage thepart.

A method is provided for thermochemically treating a part including thesteps of providing a mask comprising a body with a seat, at least adeformable sealing washer located in the seat, and a tightening bushingmoveable with respect to the body between a tightened position where thetightening bushing presses the sealing washer and a loosened position,the body having a cavity which can be accessed through a passagedelimited in the sealing washer when the tightening bushing is in itsloosened position; placing a first portion of the part in the cavitywhile the tightening bushing is in its loosened position, a secondportion of the part being located in the passage, and a third portion ofthe part being located outside the mask; moving the tightening bushingto its tightened position so that the sealing washer is deformed andapplied against the second portion of the part; and applying athermochemical treatment to the third portion of the part.

A mask is also provided including a body with a seat, at least adeformable sealing washer located in the seat, and a tightening bushingmoveable with respect to the body between a tightened position where thetightening bushing presses the sealing washer and a loosened position,the body having a cavity which can be accessed through a passagedelimited in the sealing washer when the tightening bushing is in itsloosened position.

BRIEF SUMMARY OF THE DRAWINGS

The invention and its advantages will be better understood on readingthe following description, given solely by way of example and withreference to the appended drawings, in which:

FIG. 1 is a schematic and partial view of a nuclear fuel assembly and acontrol cluster,

FIG. 2 is a schematic view of an installation for treating claddings ofneutron absorbing rods according to an embodiment of the invention, and

FIG. 3 is an enlarged and exploded cross sectional view of a mask of theinstallation of FIG. 2, the view being taken along line III-III.

DETAILED DESCRIPTION

FIG. 1 shows a part of a nuclear fuel assembly 1 and part of a controlcluster 3 to control the reactivity of the core of a nuclear reactor inwhich the nuclear fuel assembly 1 is loaded.

Conventionally, the nuclear fuel assembly 1 comprises a bundle ofnuclear fuel rods (not shown) and a skeleton 5 to support and maintainsaid bundle. The skeleton 5 includes a lower nozzle 7, an upper nozzle9, and guide thimbles 11 which connect the lower nozzle 7 and the uppernozzle 9.

One guide thimble 11 is shown on FIG. 1. The control cluster 3 hasneutron absorbing rods 13 (only one is shown on FIG. 1) and a spider 15.The spider 15 supports and maintains the neutron absorbing rods 13 sothat they are parallel and positioned laterally along the same networkas the guide thimbles 11 of the nuclear fuel assembly 1 placed under thecontrol cluster 3.

The spider 15 includes a knob 17 for connecting the control cluster 3 toa displacement mechanism (not shown) and arms 19 secured to the knob 17.One or more neutron absorbing rod(s) 13 are fixed on each arm 19.

The neutron absorbing rod 13 shown in FIG. 1 comprises a cladding 21containing at least one neutron absorbing material 23, such as bar(s) ofAg—In—Cd alloy or hafnium or hafnium-zirconium (HfZr) alloy, and/or astack of pellets of boron carbide (B₄C), of hafnium-zirconium diboride((HfZr)B₂) . . . . The cladding 21 is a tube with a circular base. Itis, for example, 3.8 m long, has a 9.70 mm outside diameter and is about0.5 to 1 mm thick. The cladding 21 is closed by an upper plug 25 and alower plug 27.

The cladding 21 and the lower plug 27 are made for example fromaustenitic stainless steels.

In other embodiments, the neutron absorbing rod 13 may consist solely ofun-clad bar(s) of neutron absorbing material such as hafnium-zirconiumalloy (HfZr) or of hafnium (Hf) e.g. with an outside diameter of around9 to 10 mm, possibly with a central hole. The neutron absorbing rod 13may also comprise such an un-clad bar in its lower portion and acladding 21 made of HfZr alloy and containing pellets of neutronabsorbing material 23 in its upper portion.

Conventionally, to adjust the reactivity of the nuclear reactor, thecontrol cluster 3 will be inserted or removed from the core of thenuclear reactor, so that the neutron absorbing rods 13 are displacedwithin the guide thimbles 11 and along corresponding guides (not shown)placed in the nuclear reactor upper internals.

The wear and corrosion resistances of the outer surface 31 of the part29 consisting of the cladding 21 and the lower plug 27 have beenimproved through a treatment.

This treatment is preferably a hardening treatment involving thediffusion of atom specie(s) into the superficial layers of the outersurface 31 of the cladding 21 and of the lower plug 27, while providinga non-treated portion of the cladding 21 for subsequent welding of theupper plug 25.

For austenitic stainless steel or nickel-based alloys for instance, itcan be any one of the treatments disclosed in the above mentionedpatents, and especially a plasma carbonitriding treatment as disclosedin EP-801,142.

The treatment is implemented in an installation 33 shown on FIG. 2. Theinstallation 33 is generally similar to the installation disclosed inU.S. Pat. No. 4,873,117.

This installation 33 comprises an enclosure 35 with a pump 37 forreducing the pressure within the enclosure 35 and an inlet pipe 39 tointroduce a specific treatment atmosphere in the enclosure 35, e.g. amix of N₂, H₂ and CH₄ when performing a plasma carbonitriding treatment.The installation 33 also comprises a generator 41 to apply a voltagebetween the enclosure 35 and the parts 29 to be treated. The enclosure35 may optionally comprise means for heating the parts 29 but such meansare not necessarily needed for achieving the treatment temperatures.Indeed, the plasma could e.g. be sufficient to achieve thesetemperatures which may be below 650° C., preferably below 600° C. andmore preferably below 500° C. for treating parts 29 made of stainlesssteel, e.g. comprised between 380 and 450° C.

The enclosure 35 comprises a mounting frame 43 for supporting the parts29 to be treated inside the enclosure 35 and electrically connecting theparts 29 to the generator 41.

The installation 33 comprises, for each part 29, a solid mask 45 forshielding an upper portion of the part 29, i.e. an upper portion of thecladding 21 from treatment.

The structure of these masks 45 is similar and only the structure of themask 45 of FIG. 3 used to protect a portion of a cladding 21 of FIG. 1will now be disclosed.

This structure is generally of revolution around a vertical axis A.

The mask 45 comprises a body 47, several sealing washers 49 and atightening bushing 51.

The body 47 and the tightening bushing 51 are for example made ofstainless steel.

The body 47 has an inner cavity 53 closed by a top end 55 and openingdownwardly outside the body 47 through an enlarged circular seat 57. Theupper end of the body 47 can be mechanically and electrically coupled tothe mounting frame 43, for example through an hole 59 cooperating with apin (not shown) of the mounting frame 43.

In the disclosed example, the mask 45 comprises a stack of four sealingwashers 49. These sealing washers 49 are rings preferably made ofrefractory material and preferably in the form of felt or plait, e.g.made of graphite felts. The sealing washers 49 are received within theseat 57.

The tightening bushing 51 comprises an inner shoulder 61 facing thelower sealing washer 49 to press the stack of sealing washers 49 axiallyagainst the surface 58 of the seat 57.

The tightening bushing 51 has an outer collar 62 comprising an internalthread 64 screwed on an external thread 48 located at the lower end ofthe body 47.

The tightening bushing 51 can be moved by screwing with respect to thebody 47 between:

an upper tightened position, where the tightening bushing 51 is closerto the body 47 and presses the deformable sealing washers 49 axiallyagainst the surface 58 of the seat 57, and

a lower loosened position, where the tightening bushing 51 is furtherfrom the body 47 and does not press the sealing washers 49.

The sealing washers 49 and the inner shoulder 61 of the tighteningbushing 51 delimit an inner circular passage 63 through which the cavity53 of the body 47 opens downwardly outside the mask 45, when thetightening bushing 51 is in its loosened position and no cladding 21 isinserted in the mask 45.

In order to treat a part 29, the cladding 21 is e.g. first provided withits lower plug 27 and optionally filled with a thermal buffer body.Then, the tightening bushing 51 being in its loosened position, theupper end of the cladding 21 is passed through the passage 63 so that afirst upper portion 65 of the cladding 21 is placed within the cavity 53of the body 47, a second portion 67 below the first portion 65 is placedwithin the stack of sealing washers 49 and a third portion 69 (FIG. 1)below the second portion 67 is outside the mask 45. As shown on FIG. 3,the part 29 preferably abuts against the top end 55 of the cavity 53.

Then, the tightening bushing 51 is screwed on the body 47 towards itstightened position. The sealing washers 49 are axially pressed againstthe surface 58 of the seat 57 so that the sealing washers 49 expandradially and inwardly to contact the second portion 67 of the part 29.

The sealing washers 49 thus seal the passage 63 and the cavity 53. Sucha sealing avoids pollution of the treatment atmosphere by the aircontained in the cladding 21 and removes the need for closing off thepart 29 with a temporary upper plug.

The part 29 is mechanically connected to the mask 45 and the mountingframe 43 through the sealing washers 49. The mask 45 and its sealingwashers 49 also ensure electrical connection between the mounting frame43 and the part 29. To that end, the sealing washers 49 are preferablymade of conductor material or of an electrical insulator material coatedwith an electrically conductive layer.

Last, the mask 45 shields the first portion 65 and the second portion 67of the part 29, i.e. of the cladding 21, from the treatment that islater applied to all accessible surfaces within the enclosure 35 andthus to the third portion 69 of the part 29.

When this treatment has been performed, the tightening bushing 51 ismoved to its loosened position and the part 29 can then be removed fromthe mask 45.

The third portion 69 has thus been treated and the first portion 65 andsecond portion 67 have been shielded from treatment and can be laterused for welding the cladding 21 to the corresponding upper plug 25.

The above disclosed masks 45 are cheap, easy to put in place and toremove and provide for an efficient shielding, even with plasmacarbonitriding treatment. The masks 45 may be reused.

The use of solid masks 45 also reduce the risk of pollution associatedwith the use of paintings as masks.

Thanks to the use of sealing washers 49 deformable by tighteningbushings 51, the risk of damaging the outer surface 31 of the cladding21 of the part 29 is reduced, despite the manufacturing variability ofthe external diameter of the cladding 21.

Other materials than graphite can be used for the sealing washers 49e.g. metallic rings, metallic or ceramic felts or plaits, compressedceramic . . . as long as they are sufficiently deformable and soft toprevent damaging the claddings 21 during contact.

Advantageously, the body 47 and the tightening bushing 51 are solidparts of resistant material to avoid thermal distortion and the thread48 of the body 47 and the thread 64 of the collar 62 arethermochemically treated to avoid galling and seizing of threads.

In a preferred embodiment, the body 47 and the tightening bushing 51 aremade of stainless steel and both of them are plasma nitrided orcarbonitrided, or at least the portions with the threads 48, 64 of bothof them are nitrided or carbonitrided before the first use. Such a mask45 can then be re-used over and over and only the sealing washers 49 mayneed to be replaced and are consumable material.

The mask 45 is fast and easy to operate. The deformable sealing washers49 compensate for the manufacturing variability of the claddings 21.

Optionally, calibrated torque wrench is used to screw the tighteningbushing 51 on the body 47. Thus, a precisely defined and uniform contactpressure is applied by the sealing washers 49 on the part 29, therebyensuring efficient holding of the part 29 only by the sealing washers 49during treatment, with low risk of damaging the geometry or the outersurface 31 of the part 29.

In the above disclosed embodiment, the mask 45 provides for mechanicalsupport and electrical connection of the part 29 but this is notnecessarily the case.

For instance, an oxidation treatment, such as the treatment taught inWO-2009/081013, does not require electrical connection of the part 29.

The parts to be treated can indeed be of other metal than steel ornickel-based alloys, e.g. zirconium, titanium or hafnium alloy.

The use of the solid mask 45 is compatible with the conditions, inparticular the temperatures of the thermochemical treatments of thesealloys, for instance from 300° C. and up to 800-1000° C. in the case ofoxidation, from 10 to 100° C. in the case of Plasma ElectrolyticOxidation (PEO) . . . .

The mask 45 can also be used with other treatments than plasmatreatments, e.g. treatments in liquid phase, or even other treatmentsthan diffusion of atoms specie(s).

Other treatments may for example be:

thermochemical treatments in liquid phase such as molten salt, e.g. forcarburizing steel parts in Borax bath. In this last instance the sealingwashers 49 may be made of alumina or zirconia felt or plait;

Chemical Vapour Deposition (CVD) treatments, e.g. forTiN—TiC—TiN—TiC—TiN deposition on niobium tubes. In this last instance,the sealing washers 49 may be made of silica felt or plait and thetemperature treatment usually from 300 to 1100° C. is compatible withthe use of the solid mask 45.

The material of the sealing washers 49 may be adapted depending on thematerial of the part 29 to be treated: silica, alumina, graphite ormetal for ionic or gas oxidation treatment, preferably graphite fornitriding . . . .

Also, the cavity 53 has a closed end in the disclosed embodiment, butthis cavity 53 can be a through hole e.g. with a sealing washerarrangement at both its openings allowing for the protection of anylocal portions of the part 29 and if needed the use of several masks 45to protect several local portions of the part 29.

In the disclosed embodiment, a stack of four sealing washers 49 is usedbut a mask 45 may comprise any number of sealing washer(s) 49, even asingle sealing washer 49.

The mask 45 and its elements may have different shapes than thosedisclosed previously. Other connections than a screw connection can beused between the body 47 and the tightening bushing 51.

In particular, the above disclosed embodiments of the invention can beapplied to parts with other shapes than tubular or bar shapes withcircular basis, in which case the body 47, sealing washers 49,tightening bushing 51 . . . may not be of revolution shapes. Inparticular, the parts can be tubular with square basis.

What is claimed is:
 1. A mask comprising: a body with a seat, the body being configured for receiving a first portion of a part; at least a deformable sealing washer located in the seat; and a tightening bushing moveable with respect to the body between a tightened position, where the tightening bushing presses the sealing washer into sealing contact with a second portion of the part, and a loosened position, the body having a cavity accessible through a passage delimited in the sealing washer for receiving the first portion of the part when the tightening bushing is in the loosened position, the body and the sealing washer being configured, in the tightened position of the tightening bushing, to mask the first portion of the part and the second portion of the part from a thermochemical treatment applied to a third portion of the part located outside of the mask, the mask having a screw connection between the body and the tightening bushing.
 2. The mask as recited in claim 1 wherein the sealing washer comprises an electrically conductive material.
 3. The mask as recited in claim 1 wherein the screw connection is formed by body threads on the body configured for engaging bushing threads on the tightening bushing.
 4. The mask as recited in claim 3 wherein the body threads are formed on an outer circumferential surface of the body and the bushing threads are formed on an inner circumferential surface of the tightening bushing.
 5. The mask as recited in claim 1 wherein the tightening bushing is movable through screwing and unscrewing of the screw connection between the tightened position and the loosened position.
 6. The mask as recited in claim 1 wherein the sealing washer is made of a refractory material.
 7. The mask as recited in claim 1 wherein the sealing washer comprises a compressed material.
 8. The mask as recited in claim 1 wherein the sealing washer comprises a plait or felt.
 9. The mask as recited in claim 1 wherein in the tightened position the cavity is configured for receiving the first portion of the part and the tightening bushing is configured for radially abutting the second portion of the part.
 10. The mask as recited in claim 9 wherein in the tightened position the sealing washer is axially compressed between the tightening bushing and the seat.
 11. The mask as recited in claim 10 wherein in the tightened position the axial compression of the sealing washer forces an inner surface of the sealing washer radially inward into gripping contact with the second portion of the part.
 12. The mask as recited in claim 9 wherein the tightening bushing includes a shoulder configured for axially abutting the sealing washer in the tightened position.
 13. The mask as recited in claim 12 wherein the shoulder axially presses the sealing washer into contact with a radially extending surface of the seat in the tightened position.
 14. The mask as recited in claim 13 wherein the shoulder is positioned inside the seat in the tightened position.
 15. The mask as recited in claim 9 wherein in the tightened position the sealing washer is compressed between the tightening bushing and the seat, in the tightened position the sealing washer being forced radially inward into gripping contact with the second portion of the part.
 16. The mask as recited in claim 1 wherein the sealing washer is configured such that the sealing washer radially contracts during loosening of the tightening bushing.
 17. The mask as recited in claim 1 wherein the cavity is closed at a first end and open at a second end, the second end extending to a radially extending surface of the seat.
 18. The mask as recited in claim 17 wherein the radially extending surface is an annular surface surrounding the second end of the cavity.
 19. The mask as recited in claim 1 wherein the sealing washer is a plurality of sealing washers in a stacked arrangement in the seat in the tightened position.
 20. An assembly comprising: a mask comprising: a body with a seat, the body being configured for receiving a first portion of a part; at least a deformable sealing washer located in the seat; a tightening bushing moveable with respect to the body between a tightened position, where the tightening bushing presses the sealing washer into sealing contact with a second portion of the part, and a loosened position, the body having a cavity accessible through a passage delimited in the sealing washer for receiving the first portion of the part when the tightening bushing is in the loosened position, the body and the sealing washer being configured, in the tightened position of the tightening bushing, to mask the first portion of the part and the second portion of the part from a thermochemical treatment applied to a third portion of the part located outside of the mask; and a mounting frame, the upper end of the body of the mask being mechanically and electrically coupled to the mounting frame.
 21. A mask comprising: a body with a seat, the body being configured for receiving a first portion of a part; at least a deformable sealing washer located in the seat; and a tightening bushing moveable with respect to the body between a tightened position, where the tightening bushing presses the sealing washer into sealing contact with a second portion of the part, and a loosened position, the body having a cavity accessible through a passage delimited in the sealing washer for receiving the first portion of the part when the tightening bushing is in the loosened position, the body and the sealing washer being configured, in the tightened position of the tightening bushing, to mask the first portion of the part and the second portion of the part from a thermochemical treatment applied to a third portion of the part located outside of the mask, the tightening bushing being movable with respect to the part and not being configured for mechanically connecting to the part. 